Coin system, refactoring, and UI update.

+ Created the purchase/coin system for towers. You get coins for killing
minions, lose them when building the tower.
+ Updated look of the main page. Created a 3 column layout, centre for
the game, right for purchasing tower pieces. Left side left unused for
now.
+ Moved some code around to be more OO:
= All loading of meshes is done in loader.js now
= Game logic put in gamelogic.js
= Only animations kept in animation.js